1. Field of the Invention
This invention relates to a method of making lightweight cement slurry and to methods of using the slurry so prepared.
2. Description of the Prior Art
When casing is cemented in a well it is a common practice to use two different cementing mixtures to fill or partially fill the annular space between the casing and the well bore. The "tail-in" mixture is placed in the bottom 100 to 1000 feet of the annulus and the "lead" or "filler" mixture is placed in the annulus above the tail-in mixture and extending upwards as far as it is desired to fill and seal the annulus.
The purpose of the tail-in mixture is to obtain a good, strong seal around the bottom of the casing and extending up over the productive zone. If the well is to be drilled deeper, the strong tail-in mixture serves the purpose of securing the bottom of the casing against the severe hammering action of the drill pipe as the well is drilled deeper. If the well has been drilled to its intended depth, the strong tail-in mixture provides a strong cement sheath that will withstand the shock of perforating the pipe and the high pressures involved in stimulating the productive formation such as by fracturing or acidizing.
The purposes of the lead or filler cementing mixture are to support the casing, prevent caving of the well bore, confine fluids to the formation in which they occur and prevent migration of fluids. For these purposes, a high strength cement is not necessary. Compressive strength of 50 psi is considered adequate for a filler slurry.
Heretofore a mixture of Portland cement and 1 to 3% anhydrous sodium metasilicate by weight of cement has been used as an economical and very satisfactory filler mixture in the cementing of wells. In addition to being economical, this mixture has a number of other advantages. The slurries can be mixed to a density as low as 11.0 pounds per gallon and yet provide acceptable strength when cured. The strength of the cured slurries is higher than the strength of most other cementing mixtures mixed to the same density. Such slurries permit a variable water ratio - the amount of mixing water used is not critical. For example, a blend of Portland cement with 2% anhydrous sodium metasilicate additive can be mixed at densities from 11.9 to more than 13.0 lbs. per gallon simply by varying the amount of mixing water.
One of the biggest advantages of the cement-sodium metasilicate mixtures is in their application to cementing on off-shore platforms. There seems to always be a shortage of storage space for dry cement on off-shore platforms. To best utilize the available storage space, it is necessary to store dry cement blends that have the highest yield. The yield of a cementing mixture is the number of cubic feet of slurry obtained per 94 pounds of the dry cement or dry cement blend. Since the known mixtures have a higher yield than other mixtures they are used extensively on off-shore platforms.
In the past, cementing mixtures have usually been made from a dry-blend of the cement and the additives needed for the particular mixture being used. This procedure has the advantage of needing only the addition of water to make the cement slurry as it is pumped into the well. However, there are also disadvantages to the use of dry-blended mixtures. If additives are dry-blended, the resulting mixture can be used only for the particular cementing job for which it was originally designed. For example, a cementing mixture designed to cement casing in a well at 10,000 feet may contain 0.5% lignosulfonate retarder to give the slurry the desired thickening time for that particular job. If it is later found that an inaccurate bottom hole temperature was used as the basis for the amount of retarder, it may then be decided that 0.8% retarder is actually needed. In this example, the procedure would be to prepare a new dry-blend of cement containing 0.8% retarder. The original dry-blend would then be wasted. For this reason it has become desirable to supply many of the additives used in cementing wells as additives that can be added to the mixing water rather than dry-blended with the cement. Additives intended for addition to the mixing water are preferably liquid rather than dry materials. Liquid additives dissolve more readily in the mixing water. Also, if a cementing mixture is to be mixed continuously as it is pumped into the well, it may be necessary to proportion the additive continuously into the mixing water. And it is much easier and more accurate to proportion a liquid additive than a dry one.
The slurries previously referred to have had a peculiar disadvantage in that the anhydrous sodium metasilicate must be dry-blended with cement. If the additive is added to the mixing water, the desired results of a uniform, non-settling slurry will not be obtained.